This invention concerns thermoforming machines of the type in which thermoplastic sheets are first heated in an oven and then transferred to a forming station where a heated sheet is drawn onto tooling forms by vacuum and/or air pressure to be formed into an article.
In some instances, a clamping frame is used to hold thinner sheets which would otherwise become too weak when heated to be transferred between stations in the machine.
When using a clamping frame, a continuous in-line process is not possible as the clamping frames must be returned to the first station and reloaded with a cut sheet.
Rotary transfer thermoforming machines are known such as described in U.S. Pat. No. 3,925,140 issued on Dec. 9, 1975, which use continuous rotation of a carrier member to carry the frames through the station and to return the frames to a load/unload station after successive rotation through the heating and forming stations.
For very large articles, rotary transfer thermoforming machines occupy excessive plant floor space.
An in-line shuttle transfer has also been used, but this either is too slow if a single clamping frame is used, or requires two molding stations on either side of an oven.
Another problem is encountered in thermoforming parts with substantially angled deep portions, as excessive stretching of the material would occur if the sheet is simply formed onto the tool shape.
Draping techniques are sometimes employed for closed shapes forming dished containers, with vacuum (or air pressure) causing a pronounced sagging of the sheet material preparatory to the forming step. Such technique is not useable with a nonclosed shape or with angled ends connected with a flat center section.
U.S. Pat. No. 4,744,848 issued on May 17, 1988, describes a frame with hinged side portions which floats to allow forming with minimal stretching. This arrangement relies on engagement with a male and female mold to cause hinging of the frame ends which in turn bends the sheets into closer conformity with the mold contours prior to forming. See also U.S. Pat. No. 3,574,807 issued on Apr. 13, 1971, which describes a frame hinged in the middle to form an angled article shape.
These patents do not disclose an adaptation of a hinged clamping frame to automated thermoforming apparatus.
Accordingly, it is an object of the present invention to provide a linear transfer system for a plurality of clamping frames used in a thermoforming apparatus to hold sheets during heating and forming steps.
The system is adapted to article sizes too large for practical application to rotary machines.
It is another object to provide a frame having a section pivot capability for sharply angled elongated parts which is adapted to an automated system and which does not require engagement with a male and female mold contour to be actuated.